The number of persons with asthma in the United States has more than doubled in the past 20 years. In 2001, the lifetime prevalence of asthma in children five to 17 years of age was 144.2 per 1,000 persons. Exposure to tobacco smoke has been shown to exacerbate asthma and increase school absenteeism in children with asthma. Recent studies have shown that maternal smoking is associated with increased risk of new-onset asthma in children. The Children’s Health Study (CHS) showed that in utero exposure to maternal smoking was independently associated with increased asthma risk in children. However, the authors could not determine how prepregnancy smoking, smoking cessation, or smoking intensity affected the risk of childhood asthma. Li and colleagues assessed the association between maternal and grand-maternal smoking before, during, and after pregnancy and the risk of childhood asthma.

The case-control study included children from the CHS who were diagnosed with asthma within the first five years of life. The participants were matched on grade, sex, and community of residence to make up the control group. Whenever possible, information was obtained through a telephone interview with the child’s biologic mother. Information was obtained on smoking status and the amount smoked before and during each trimester of pregnancy. Smoking intensity was defined as the most common amount smoked during the three trimesters. Secondhand smoke was determined by the relationship to the child and number of smokers inside the house daily. Transgenerational exposure to maternal smoking was assessed by asking the child’s mother, “Did your mother smoke when she was pregnant with you?” Additional information was gathered about history of asthma in parents and siblings.

The number of children in the study who had asthma was 338, and there were 570 control children identified for comparison. Exposure to maternal smoking in utero was associated with an increased risk of developing asthma during the first five years of life (adjusted odds ratio [OR] 1.5; 95% confidence interval [CI], 1.0 to 2.3). This exposure also increased the risk of persistent asthma during the first five years (OR 1.5; 95% CI, 1.0 to 2.3). When compared with mothers who never smoked, those children whose mothers smoked during the pregnancy had a higher risk of developing asthma during the first five years of life. Children of women who stopped smoking before the pregnancy did not have an increased risk of developing asthma when compared with those who had never smoked. Grandchildren of women who smoked during pregnancy had an increased risk of developing asthma.

The authors conclude that maternal and grandmaternal smoking during pregnancy may increase the risk of childhood asthma. Smoking cessation in women before pregnancy, however, can decrease the risk of childhood asthma.